The present invention relates generally to an improved apparatus and cost-effective method for preparing printed circuitry including copper plated polymer thick films arranged on a substrate having conductive bridges formed thereon to electrically couple discrete electrical conductors arranged on opposed surfaces of a common substrate, and with the conductive bridge being characterized in that it extends uniformly along and through the walls of a bore formed in and through the common substrate. More particularly, the present invention relates to an improved method and apparatus for preparing improved dual-sided circuitry coupled through conductive bridges which electrically couple discrete electrical conductors by means of an improved conductive bridge extending along and through the inner wall of a bore formed through the common substrate as a substantial uniform continuation of the respective opposed electrical conductors. In order to complete preparation of the improved conductor arrangement of the present invention, the following sequence of operations is normally undertaken:
1. A conductive ink in the form of a polymer thick film is screen printed onto opposed surfaces of a pre-punched substrate utilizing polymer thick films prepared and cured in accordance with the present invention;
2. Electroplate all surfaces of the cured conductors with a film of a conductive metal such as copper, nickel or other suitable metal or platable metal or alloy. Alternatively, electroless plating may be employed, even though electroless plating may have a much slower deposition rate;
3. Screen-print a dielectric ink pattern upon the metallic coating, such as copper or nickel, leaving via openings wherever an interconnect is required;
4. Screen-print a second layer of polymer thick film onto both surfaces, and effect an interconnection to the metal-plated circuit i.e. copper or nickel through the vias; and
5. Electroplate both of the opposed surfaces with a film of metal such as copper or nickel.
Through the utilization of this process, multilayered dual-sided circuitry having exceptional electrical and physical properties may be fabricated.
Plated-through holes are widely employed as a method of interconnecting conductors disposed on different planes of a substrate, particularly conductors disposed on opposite sides of a pre-punched common substrate, as well as conductors disposed on separate or individual substrates. Accordingly, plated-through holes frequently form the conductive bridge between such conductors or conductive patterns. In the past, various techniques have been used to provide electrically conductive bridges or interconnects on a through-hole basis. While attempts have been made to successfully form such conductive bridges utilizing a screened through-hole process, such attempts have previously proved to be impractical with results that prove unreliable. In accordance with these prior efforts, a curable liquid conductive ink coating was typically applied to the substrate, and printed in sufficient quantity so that a portion of the conductive coating would be designed to extend on a uniform basis integrally through the bore so as to coat the wall surfaces and provide the conductive bridge performing the electrical interconnect function. In order to increase reliability, redundant openings as well as separate mechanical interconnects have typically been utilized in the past to provide reliable and durable interconnects extending from one side of the substrate to the other. The screened through-hole process of the present invention essentially eliminates the need for redundant interconnects. Additionally, the copper plating of the polymer thick film pattern makes it possible to fabricate multilayered circuitry with interconnects extending through from one plane to another, including interconnects between adjacent planes or layers of circuitry, as well as interconnects extending across bores formed in the substrate.
In order to reduce space and volume requirements, and also to provide for appropriate density of circuitry, it is, of course, desirable if not requisite to employ double-sided circuitry on flexible substrates. Multilayer circuitry enhances the utility and versatility of such circuitry. Typically, one circuit pattern will be initially delineated on one surface of the substrate by silk screen printing of a curable liquid conductive ink coating, with the conductive ink coating being curable through exposure to an external energy source such as ultraviolet radiation and/or heat. Such conductive inks as well as their curing capabilities when cured are frequently referred to as "polymer thick films", and are well known and in common use in the printed circuitry art, and are, of course, commercially available. Following printing and curing of the first conductive ink pattern, a second conductive ink pattern is then formed on the opposed surface of the substrate. The substrate, having conductive patterns disposed on both major surfaces, is capable of functioning as one or more conductive patterns or conductors in an assembly or array of conductors. Typically, utilizing the features of the present invention, conductors prepared from polymer thick films having a thickness of from about 0.3 mil up to about 3-4 mils are easily and readily prepared. Material for the substrate, while not critical to the operation, is preferably flexible and fabricated from stress-oriented polyethylene terephthalate and having a thickness that may range from between about 1 mil and 20 mils, with a thickness range of between 2 mils and 10 mils being generally preferred. At greater thicknesses, such as thicknesses greater than about 20 mils, certain operations in the process must be run at a slower rate in order to achieve appropriate and desirable cures. Such substrate materials are, of course, commercially available.
In order to preserve the integrity of the circuit pattern, and in order to appropriately provide for preparation of a coating delineated as a circuit pattern on the flexible substrate, a supply web of a generally impervious flexible substrate (stress-oriented polyethylene terephthalate) is provided with a number of bores which are formed in the substrate so as to accommodate electrical interconnects where desired. For preparation of a first conductive pattern along one surface of the flexible substrate supply web, this web is initially superimposed over a second substrate, the second substrate being a generally porous, flexible non-woven web material which in the present operation, functions as a vacuum diffusing carrier web. This non-woven flexible web is pervious to air, but impervious to curable liquid conductive inks, thereby adapting its use toward that of a vacuum diffusing carrier web in the method and apparatus of the present invention.
In accordance with further features and operations of th present invention, the superimposed web pairs are moved along a path within a substrate printing zone wherein the webs traverse at least a portion of the arcuate periphery of a cylindrical printing pad having an outer annular wall surrounding a hollow core coupled to a source of vacuum. The outer annular wall is perforated with a plurality of bores to provide a perforate rotating printing cylinder for carrying the web pair at a predetermined peripheral rate of speed thereacross. While the web pair traverses the surface of the perforate cylindrical printing pad, a layer of curable liquid conductive ink is applied to the surface of the substrate through a superimposed silk screen, with the applied vacuum tending to draw the conductive ink coating through the substrate, and with the air-pervious vacuum diffusing carrier web trapping the conductive ink and thus controlling its movement, travel, and ultimate disposition.
The porous non-woven flexible vacuum diffusing carrier web is typically fresh filter paper or other porous fabric, with the material having a preferred thickness of between about 5 mils and 8 mils, and a weight of about 1.28 ounces per square yard. Such filter papers are, of course, commercially available and have been found to function well as a flexible vacuum diffusing carrier web pervious to air but impervious to curable liquid conductive inks. As a follow-up to the preparation of the first conductive pattern along one surface of the flexible substrate supply web, the operations undertaken for the preparation of the first conductive pattern are repeated on the opposed surface of the substrate supply web so as to form the second conductive pattern on the opposed major surface of the web.
In order to provide multilayer capability on the previously coated web, a film or layer of copper, nickel or other suitable metal is electroplated simultaneously onto both surfaces of the substrate. Thereafter, a dielectric film is printed upon the copper plating with vias formed through the dielectric film so as to permit coupling to the copper-plated layer therebeneath. Following completion of the formation of the dielectric film, a second polymer thick film is coated upon the substrate, this second film being applied directly over the dielectric film layer just formed. Thereafter, the outer surface of the second polymer thick film may be plated with copper, nickel or other suitable metal, thereby providing a highly durable and highly reliable multilayered dual-sided circuit. The final oouter layers of electroplated metal may be further plated with a non-oxidizing or nobel metal such as gold or possibly silver.